Heating of functional units (or as used sometimes herein “heatable components”) on motor vehicles are electric where heating resistances are fed from the battery or generator (alternator) or are heated by the engine through the air. Heating a heatable component such as a side mirror, a lock or a windshield of a vehicle is usually undertaken by at least one electric heating element whose heating power can be controlled electrically by for example an operator switch.
EP 0 408 853 A2 teaches heating a vehicle side mirror where, for heating, the current flow through a heating conductor is controlled by a semi-conductor switch. The semi conductor switch is controlled through a temperature sensor and a two-stage amplifier circuit which behaves like a Schmitt trigger. The semi conductor switch thereby forms one of the two stages which are coupled together for the Schmitt Trigger behavior. The drawback with this solution is that when the temperature drops below 27° C. the heating current is switched on until a temperature of 30° C. is reached even if heating is not necessary for a clear view of the mirror surface. The amount of energy required for the heating device for the mirror glass is therefore increased unnecessarily.
DE 197 05 416 C1 teaches a method for controlling the heating of a rear windshield of a vehicle where the heating of the rear windshield is switched off at least after a certain switch-on time. The certain switch-on time of the rear windshield heater is extended as the drive speed of the vehicle increases. This extension of the switch on time can also lead to strain on the on-board power supply or vehicle battery without any benefits to the vehicle occupants.
In DE 91 08 801 U1, a voltage drop which is dependent on the temperature of the mirror glass is compared by a comparator with a reference value and a switch of the comparator is controlled based on the result of the comparison. The heating current for this purpose is compared with a reference value. A control device containing the comparator for heating the mirror glass on a vehicle side mirror is provided with a heating resistance which can be switched to a current source by means of a switch. The comparator detects the voltage drop at a resistance through which the heating current flows and compares it with a reference value. The switch of the comparator is controlled depending on the result of the comparison. The use of the temperature path of the specific resistance of the heating resistance is based on the fact that the temperature of the heating resistance, which rests with its full or partial surface on the mirror glass, corresponds roughly to a mean value of the temperatures of the different mirror glass regions, when the heating current is interrupted. A high set reference value or a large manufacturing tolerance of the heating resistance leads in turn to a poor energy utilization of the vehicle battery.
Therefore, a method or device for heating a functional unit on a motor vehicle which reduces the energy consumption required by the heating would be beneficial.